This subproject is one of many research subprojects utilizing the resources provided by a Center grant funded by NIH/NCRR. The subproject and investigator (PI) may have received primary funding from another NIH source, and thus could be represented in other CRISP entries. The institution listed is for the Center, which is not necessarily the institution for the investigator. The interferon-induced host protein BST-2 (also known as HM1.24, CD317, and tetherin) has recently been found to function as an antiviral factor that restricts the release of newly formed virus particles from infected cells. HIV-1 encodes a specific protein, Vpu, that antagonizes this restriction. The ultrastructural distribution of BST-2 in relation to specific cellular membrane systems and forming virus particles is unknown. We intend to determine this using correlative light and electron microscopic immuno-localization techniques. These experiments are expected to reveal aspects of the mechanism by which BST-2 restricts viral replication and the mechanism by which HIV-1 counteracts this restriction.